Yttria (yttrium oxide, Y.sub.2 O.sub.3) is used commercially in a variety of applications because of its chemical stability and refractory nature. A refractory material is any of various substances, such as ceramics, that are characterized by their suitability for use as structural materials at high temperatures. A ceramic is a hard, brittle, heat-and corrosion-resistant material that may be produced by forming the ceramic particles into a desired shape and then firing the shape to its final density. Yttria also is used extensively as a component for industrially important ceramics, such as zirconia and silicon-nitride based materials and high T.sub.c superconductors.
Aqueous suspensions of yttria particles are used industrially to form ceramic articles. A suspension is a system in which small particles, typically solid particles, are uniformly dispersed in a liquid, such as water. Ceramic suspensions that are used commercially to make ceramic objects commonly are multi-component compositions. For instance, commercial ceramic slurries typically include processing aids, such as dispersing agents, co-solvents and binding agents. Ceramic slurries also may include materials that are to be incorporated into the final ceramic product, such as strengthening agents, stabilizing agents, and sintering aids.
Aqueous ceramic processing is preferred for processing ceramics, primarily because aqueous processing is cheap and relatively environmentally safe. Aqueous processing of some ceramics is difficult because ceramic materials normally are at least partially soluble in water. Furthermore, ceramics hydrate in aqueous systems, which means that the ceramic particles react with water to form a chemical bond. The compounds that result from the hydration are referred to as hydrates. If hydration and/or dissolution of the particles occurs extensively in aqueous environments, then subsequent aqueous processing becomes difficult or impractical.
Ceramic particles also tend to agglomerate in water. Agglomerated suspensions are not useful for most applications, and must be disposed of at considerable expense and loss of material. The extent and rate of ceramic dissolution, hydration or agglomeration in water depends on many factors, including the nature of the ceramic, the oxidation state of the ceramic, the pH of the system and the temperature of the system.
For commercial applications, the properties of a slurry preferably do not change over time. Persons skilled in the art of ceramic processing continually seek methods for forming colloidal ceramic suspensions that are stable for relatively longer periods of time. That is, persons skilled in the art have sought methods for preventing particle agglomeration, while simultaneously reducing the dissolution and hydration rates.
Previous known attempts to make low-pH, aqueous yttria slurries that are stable for periods of up to about a week have been unsuccessful. For instance, Lassow's U.S. Pat. No. 4,703,806 states that an aqueous slurry of yttria and colloidal silica gels prematurely. Once the slurry gels, it is no longer useful for ceramic processing.
As another approach, Horton's U.S. Pat. No. 4,947,927 recites that an aqueous yttria slurry having a colloidal silica binder does not gel prematurely as long as the slurry has a pH of at least 10.2, and preferably greater than about 11.0. Horton specifically recites that a slurry having a pH of less than 10.2 experienced premature gelation after only six days. As illustrated by FIG. 1, a small drop in the pH from 11 to only 10.5 will increase the concentration of dissolved yttrium by more than one order of magnitude, and may cause premature gelation of the slurry. Maintaining the pH of yttria slurries in a production environment above 11 at all times creates production difficulties. Furthermore, compositions exhibit increased toxicity as the pH varies significantly from a neutral pH value. For these reasons, it is preferable to make yttria slurries less pH sensitive and at pH values less than about 11, in direct contrast to the teachings of Horton.
Another known approach for reducing the aging of yttria slurries is to reduce the concentration of dissolved yttrium ions by removing such ions from the slurry as they dissolve. This approach is relatively expensive and inefficient for high-volume slurry applications. An example of this approach is disclosed by G. M. Crosbie, "Ion-Exchange Treatment of Silicon-Yttria Dispersions" J. Am. Ceram. Soc., 68 [3] C-83-4 (1985). Crosbie kept his silicon-yttria suspension stable for at least 24 days using an ion-exchange process which involved exchanging the dissolving polyvalent ions with monovalent ammonium ions.
In summary, a need exists for an improved aqueous yttria-containing slurries which resists gelation over time.